1. The Field of the Invention
The present invention generally relates to stationary anode x-ray tubes. In particular, the present invention relates to structures and methods for controlling the unintended emission of x-rays from certain regions of a stationary anode x-ray tube, thereby decreasing the need for external tube shielding.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in applications such as diagnostic and therapeutic radiology, semiconductor fabrication, joint analysis, and non-destructive materials testing. While used in a number of different applications, the basic operation of an x-ray tube is similar. In general, x-rays are produced when electrons are accelerated and impinged upon a material of a particular composition.
An x-ray generating device typically includes a cathode having an electron source, and an anode disposed within an evacuated enclosure. The anode includes a target surface that is oriented to receive electrons emitted by the electron source. In operation, an electric current is applied to the electron source, such as a filament, which causes electrons to be produced by thermionic emission. The electrons are then accelerated towards the target surface of the anode by applying a high voltage potential between the cathode and the anode. Upon striking the anode target surface, some of the resulting kinetic energy is released as electromagnetic radiation of very high frequency, i.e., x-rays.
The specific frequency or wavelength of the x-rays produced depends in large part on the type of material used to form the anode target surface. Anode target surface materials with high atomic numbers (“Z” numbers), such as tungsten, are typically employed. The x-ray ultimately exit the x-ray tube through a window in the x-ray tube, and interact in or on a material sample, patient, or other object. As is well known, the x-rays can be used for sample analysis procedures, medical diagnostic and treatment, or various other applications.
Many x-ray tubes employ a rotary anode that rotates portions of its target surface into and out of the stream of electrons produced by the cathode filament. However, in other tubes a stationary anode is used. The anode in stationary anode x-ray tubes typically includes a substrate portion, comprised of copper or similar material, and a target surface comprised of rhodium, palladium, tungsten, or other suitable material. The target surface is angled toward the tube window to maximize the number of x-rays produced at the target surface that can exit the tube.
Notwithstanding the angled orientation of the stationary anode target surface, x-rays nonetheless emanate in all directions from the target surface after their production. Thus, while a portion of the x-rays does indeed pass through the window to exit the tube and be utilized as intended, a large number of x-rays do not. X-rays that do not pass through the window penetrate instead into other areas of the x-ray tube and can escape the tube if sufficient measures to prevent their escape are not taken. Escape of such non-window transmitted x-rays from the tube is highly undesired as they can represent a significant source of x-ray contamination to tube surroundings. For instance, users of an x-ray tube that emits undesired x-rays through non-window tube surfaces can receive relatively high doses of x-ray radiation, which can result in adverse health effects. In addition, such non-window transmitted x-rays can interfere with the primary x-ray stream that is properly transmitted through the window, causing reduced quality results. In x-ray imaging, for example, non-window transmitted x-rays from the x-ray tube can impinge upon areas of an object to be imaged and interfere with the image being sought. The interference caused by the impingement of the undesired x-rays is manifested as clouding in the image, thus reducing image quality.
Efforts to reduce the emission of x-rays from non-window portions of an x-ray tube have centered around the use of external shielding on tube structures. For instance, in many stationary anode tubes a layer of lead shielding is placed about the inner surface of an outer housing that contains the tube to absorb non-window transmitted x-rays that are produced at the target surface and penetrate the tube's evacuated enclosure.
Despite its utility in preventing undesired x-ray emission from the x-ray tube, lead linings nevertheless suffer from a number of challenges. Primary among these is the fact that, though effective at absorbing x-rays, lead is relatively heavy and substantially adds to the weight of the tube. This factor becomes important in applications where a relatively low tube weight is desired or even required. In addition, because the lead lining is placed relatively far away from the target surface of the anode (i.e., attached to the outer housing located beyond the outer surface of the evacuated enclosure), large amounts of lead must be used to cover relatively large portions of the enclosure surface to account for the radially expanding pattern of x-ray emission from the target surface. Indeed, nearly the entire surface area of the evacuated enclosure is covered by lead lining to prevent x-ray emission from the tube. The addition of lead linings described herein represents a significant cost in time and labor during x-ray tube manufacture.
It is further known that certain areas of the x-ray tube are especially susceptible to the impingement of non-window transmitted x-rays. These areas include one or more ports defined in the outer housing through which high voltage cables pass to provide a voltage potential for the cathode, anode, or both. In an anode grounded x-ray tube, for instance, a voltage supply is provided to the cathode via a high voltage cable that passes through a port defined in the outer housing and electrically connects with a portion of the cathode. Because of electrical insulation requirements between the cathode and the high voltage cable connection thereto, adequate x-ray shielding is difficult to attain near the port. Specifically, lead shielding, which is electrically conductive, cannot be disposed near the high voltage connection between the cable and the cathode so as to maintain the electrical isolation of the cathode. Thus, x-rays that would otherwise be absorbed by lead shielding are instead allowed to pass through the high voltage connection area and exit the port, thus providing a contamination point through which significant x-ray escape from the tube can occur. If left unchecked, this unintended x-ray emission can compromise tube performance and damage the near-tube environment. At the very least, this situation requires the placement of additional shielding around the tube to absorb any x-ray emission from the port, undesirably adding weight to the tube.
In light of the above discussion, a need exists in the art for a means by which unintended x-ray emission from an x-ray tube is prevented. Additionally, any such means should minimize the use of excessive, heavy external shielding that significantly adds to the (weight of the tube. Any solution to the above problems should additionally provide for a relatively light x-ray tube that enables its use in weight-sensitive applications.